Live - line surge arrester installing on a mountain

section of the 400 kV Gutinas- Brasov o/h line (between towers 94-100)

G. A. Florea, S. Member, IEEE, D. Marginean, M. Oltean, C. Chiriac, I. Rodean, A. Mazilu, C. Oprea

• Introduction• The 400 kV Gutinas- Brasov was designed and erected during the

period 1970- 1974. The line is crossing the East Carpati Mountainswhere the risk of high thickness of ice on conductors is to be meteach winter. Initially, the line was provided with 2 shieldwires overallits length, but later due to the occurred failures of the shieldwires intwo sections of the line, the shieldwires were dismantled. Thesesections were left to the direct strikes of the lightning. Therefore,later, after more than 30 years, in order to diminish the outages ofthe line due to the lightning phenomenon, based on the internationalexperience in the field, the solution of surge arresters mounting ofthe involved towers was achieved. This paper presents the matter ofthe installed arresters of the section between the towers 94 and 100.

• The measurements of the earthing resistance showed that some ofthe towers of the mentioned section are not corresponding, theresistances being of more than the maximum allowed 25ohms andtherefore solutions for the grounding improvement had to be takenas well. In the mentioned line section two types of towers are met:the Y suspension tower (Fig. 1) and the monophase tension towers(fig. 2).

Fig. 1. Suspension Tower- photo

Fig. 2. Tension Tower- photo

• ELECTRICAL AND MECHANICAL CONDITIONS FOR ARRESTER INSTALLMENT

• A. The main characteristics of the arresters• From the two main solutions of arrester mounting, with rigid

insulating connection of the hot part of arrester (normal conditions)and the hot part of the line (conductors, clamps) or with no rigidconnection, the last one was chosen for this project.

• B. The clearance criteria• The arrester locations have to be carefully found based on a right

analysis of the clearances between the hot parts and the groundedparts, both under high wind or in still air. The asynchronousmovements of the suspension insulator strings and arresters have tobe considered too. The dynamic pressure of the wind in the area is45 daN/sq.m in accordance with the Romanian standard NTE003/004/00.

• The minimum required distances in the air between the hot andgrounded points are 2900 mm in still air and 1000 mm in the case ofmaximum wind with no ice on conductors. Practically, thesedistances have to be observed between the arrester bottom terminalor its corona ring and the hot points as; conductors, clamps, etc.

• In the case of arrester, installing on old lines it becomes almostimpossible to respect the required distances in all the cases. To fulfillall the requirements is needed to perform important changes in thetower geometry involving long time line operation interruptions.Since the arresters have a deffect rate of 2x10-5 only, it has to beassumed the risk of line operation interruption for a time ofmaximum 10 hours to allow the maintenance team to dismantle thedeffected arrester.

C. Mechanical criteria• Since different crossarms have to be mounted on the existing

towers to allow the installment of arresters at establisheddistances from suspension insulators strings either in the case ofsuspension towers or in the case of tension towers, the towerelelements have to be recalculated taking into account the newforces and moments involved by the new elements.

• THE PROPOSED SOLUTION• A. Arresters • The chosen arresters had to satisfy the requirements of the

international standards as; IEEE C62.22, IEC 60099, IEC 60694,IEC 60060 and IEEE 1243, corresponding to the nominal voltage ofthe system; 400 kV. The arresters are of composite type of insulation.

• B. Arrester positioning• The deflection angle of the arrester under maximum wind may be

calculated taking into account the weight of arrester of 138 daN,from which 38 daN being the weight of the corona ring. The positionof the maxium deflected point of corona ring is of 2.338 m from thevertical axis of the arrester in still air.

• The suspension insulator string can not be deflected either than intransverse plane on the line axis, in normal conditions and themaximum angle is 600. In the case of the suspension insulator stringof the jumpers, for towers 94 and 100 (tension monophase towers)this angle is 300.

• In the case of the asynchronous movement of the arresters andsuspension insulator strings, taking into account the arresterdeflection angle of 250, is needed a horizontal distance of 2300 mmbetween the both axis.

• Finally, for suspension towers a longitudinal space between thearrester axis and suspension insulator string axis of 2300 mm had tobe considered. Also, a transverse space of 2000 mm had to beconsidered too. In the case of arrester damage, the bottom terminalcoming to the ground voltage, a distance of 1000 mm will be keptfrom this one and the hot points of the phase, which is quite OK, ifno wind at all (Fig. 3, 4, 5).

• For the monophase towers similar distances are considered (Fig. 6).If the arrester fails, the distance between the arrester bottom withground voltage and the phase is 1474 mm, allowing the lineoperation, if no wind or low value of wind velocity will occure.

Fig. 3. Suspension Tower - up to down view drawing

Fig. 4. Suspension Tower- center phaseadditional crossarm- up to down viewdetailed drawing

Fig. 5. Suspension Tower- outer phaseadditional crossarm- up to down viewdetailed drawing

Fig. 6. Tension Tower- up tp down view drawing

• TOWER EARTHING RESISTANCE IMPROVEMENT• The earthing resistances in the mentioned line section were

measured, the results over 25 ohms being: 95, 96, 97, 98- over thescale, 99- 25, 43 and 25, 39 ohms, 100- 2 ohms. The soil resistivityin the area was measured too, resulting: 94- 95:27,9 ohms, 95- 96:501 ohms, 96- 97: 554 ohms, 97- 98: 1450 ohms, 98- 99: 127 ohms,99- 100: 519 ohms.

• In the case of soil, electric resistivity over the value of 500 ohmmthere may be allowed tower earthing resistances of maximum 30ohms, but in the case of arresters, the maximum accepted value is25 ohms.

• Since the improvement of the existing earthings by additional verticalrods and/or tape, type horizontal electrodes cannot be done due tothe impossibility of the rod driving into the very hard rocky soil andamplifying the existing earthing by additional tape figures leads to theefficiency reduction it was proposed a „counterpoise earthingsystem.” This sytem consists of horizontal electrodes connected tothe tower. These electrodes may be radially laid in soil around thetower, with individual lengths less than the maximum efficient lengthor to connect all the towers of that section by an horizontal electrode,along the line route.

• The electrodes may be made by steel, aluminum or copper, thecross section having to correspond to short-circuit current.

• The following equations may be used for the resistance calculation:• Formula 1 - E. D. Sunde, 1968 [1] – for the towers with one

horizontal connection only;

−⋅⋅

⋅⋅

= 12

2ln

da

l

lR

πρ

for d<<1 (1)

ρ - electric resistivity of the soil, in Ωml – length, in ma – conductor radius, in md – depth, in mσconductor = ∞

• Formula 2 -J. G. Anderson – EPRI- second edition [2] taking into account the electric resistivity of the electrod:

⋅⋅=

ρρ r

lrR coth0

ρ - electric resistivity of the soil, in Ωml – length, in mr – conductor resistance, in Ω/m

(2)

• Formula 3- E. D. Sunde, 1968 [1] – for the towers with double connections;

−⋅⋅

⋅⋅⋅

= 3.02

2ln

2 da

l

lR

πρ

pentru d<< l (3)

ρ - soil electric resistivity, in Ωml – length, in ma – conductor radius, in md – depth, in mσconductor = ∞

Applying the above equations were got the diagrames of the Fig. 7.

Fig. 7. Diagrams of the earthing resistance

• But, the calculation results may be affected by the effective length,the maximum length, over which any prolongation of the length doesnot bring any improvement of the resistance. To calculate thiseffective length the equation of Jinliang He, 2005 [3] may be applied:

( )097.0

379.0528.6

I

Tlcef

⋅= ρ

=

=

kA

kAI

s

sT

20

106.2

2.1

µµ (4)

mΩ= 15001000500ρ

In Fig. 8 may be observed the results. It has to be mentioned theinfluence of T- wave front of the overvoltage and I – the maplitude ofthe current wave.

Fig. 8. Diagrams of the effective length of the counterpoise earthing

• The counterpoise system will be used for the towers 95- 99, usinggalvanized steel tape of 40 x 4 mm. the effective lengths are of 112m for soil of 1500 ohmm and of 74 m for soil of 500 ohmm. All thetowers 95- 99 will be electrical connected by this system.

• LIVE MOUNTING OF ARRESTERS• Since the 400 kV OHL Gutinas- Brasov is an important line it is very

difficult to be deenergized. That was the reason to adapt liveworking technologies.

• A. General Circumstances• For practical works the Procedure NTE 010/11/00 – ”Technical

Normative Establishing the Requirements for Live Working inElectrical Networks” and working procedures of Smart [4, 5] shouldbe applied.

The minimum number of staff required for this work is:•one LW foreman (team leader), authorized I 3T,•six (6) LW workers, authorized I 1T.

The method applied is “bare hand” (the worker finds himself at thesame potential as the conductive elements on which he is working)

• When the works have to be done it is necessary for the equipmentto be in special exploitation regim (RSE) which consists in:

• autoreclosing cancelled,• radio or telephone links have to be established between the

working area and operative dispatch center,• the reclosing of the OHL should be done manually, only when

the LW foreman agrees.

• B. Weather Conditions• The LW foreman has to analyze the weather conditions. It is

forbidden to start the work under the following circumstances:• Rainfalls – Rainfalls refer to rain, snow, hail, drizzle, rime or

humidity of more that 80%,• Thick fog – It is considered to be thick fog causing humidity

of more than 80% or which dangerously reduces work safetyvisibility,

• Reduces visibility – The situation in which the LW foremancannot clearly distinguish the workers in his team or the liveelements the team has to work on,

• Electrical discharges – Are considered to be lightning orthunders occurred at the workplace,

• Strong wind – Strong wind is considered to be the windwhose speed in the working area is more than 9,5 m/s.

• C. Risk Assessment and Prevention Measures• The LW foreman should have on him the LW Authorization filled-in

for the specific working area, the Technical Instructions (SMART –IT– LST 10/2011) and the Specific Work Safety Instructions (SMART-ISSM – 10/2011), related to this work.

• All along the working period, the foreman has to supervise theworking team and the working area.

• D. The Working Procedure• The foreman identifies the OHL and the tower where the work has to

be carried on. He checks the humidity level of the air, the windspeed and decides wether the work should be started. He ensuresthat every crewmember is well instructed and understood theseinstructions.

• The crewmembers that will work on the tower are supposed to wearthe electro conductive suits, anti-UV glasses, electro conductiveboots, protective helmet, climbing system, the little roll and insulatingrope.

• The electro insulating tools are placed on the tarpaulin and on thetool’s props. Before the work starts, the rods shall be siliconated.

• The workers are climbing on the tower.• The arrester is lifted up to the tower’s crossarm and it is connected

to the tower.

Fig. 9. The arrester is prepared on the ground

• The chair assembly is placed on the crossarm, together with theneeded insulating ropes.

• The chair is pulled close to the tower. One of the worker passesfrom the tower to the chair. He ensures himself, using the ensuringrope and unties the rope that is tightening the chair to the tower.

• The worker sitting on the chair is pulled, from the ground, using aninsulating rope, in vertical position, close to the active wire.

• Note: The worker should avoid any movement while the chair is inmotion, until he reaches the working position, as this may cause theexceeding of the chair’s limits.

Fig. 10. The worker is connected to the potential

• When the distance is convenient the worker should place himself tothe active wire potential, using the potential rod. The potential belt isconnected to the active wire.

• The chair is moved to the arrester. The second end of the potentialbelt is connected to the arrester. The worker is taking the arrester’swire and moves again in the working position. The active wire isrubbed and prepared for the connection. The clamp is connected tothe active wire.

• Once the work is finished, the workers dismantle the used tools. Theaction is the same with the initial one, only reversed.